Impedance measurement in an active radio frequency transmitter

ABSTRACT

A transmitter and method for determining impedance measurement in an active radio frequency transmitter of an antenna are provided. Voltage and current samples are obtained from a modulating signal delivered to an antenna. The voltage and current samples are converted from analog to digital format, and decimation occurs to reduce the number of samples. Complex demodulation is performed of each of the voltage and current samples to baseband levels and decimation of the voltage and current samples is performed to reduce the number of samples. An impedance estimate can then be estimated from the voltage and current samples.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/421,519 filed on Apr. 9, 2009, which further claims benefit of U.S.Provisional Patent Application No. 61/044,213 filed on Apr. 11, 2008.The disclosures of the foregoing patent applications are herebyincorporated by reference herein in their respective entireties, for allpurposes.

TECHNICAL FIELD

The present disclosure relates to impedance measurement in a radiofrequency transmitter. More specifically, it relates to a method forimpedance measurement in an active, or running, radio frequencytransmitter of an antenna coupled to the transmitter.

BACKGROUND

For radio broadcasters, knowing the impedance a transmitter is operatinginto can be essential for ensuring proper spectral performance andreception, particularly with digital standards such as National RadioSystem Committee NRSC-5A In-band/on-channel Digital Radio BroadcastingStandard. The impedance may be influenced by different factors such asthe transmitter filter, tuning networks external to the transmitter, andthe antenna. As a result, the impedance must fall within certainspecifications, or problems may develop with respect to spectralcompliance or reception which is especially true for digital operation.

Currently, the equipment required to determine the impedance that thetransmitter is operating into has to be inserted at the appropriateplace, so the transmitter must be disconnected to effect thismeasurement. This results in the transmitter being temporarily taken offthe air while the measurement is being made. In addition, specializedequipment that may not be commonly available at the transmitter site isrequired to make the measurement. As a result of these factors, thecurrent method is time consuming, costly and can require severaliterations before an acceptable operating impedance is reached.

For broadcasters operating in a single frequency network, using atraining signal to measure the impedance is not a preferred option,since emitting a tone would disrupt the other transmitters in thenetwork. This method virtually eliminates reception, therefore, thebroadcaster's only option for measuring the impedance involvesdisconnecting the transmitter to use low power test equipment. Inaddition a generated test signal is typically utilized to performimpedance measurements for matching network tuning. These estimates maynot accurately characterize the impedance response based upon broadcastsignals propagated by the antenna providing less than optimized tuningof the matching network.

The current methods of measuring the impedance seen by a transmitterrequire disconnecting the transmitter, which is inconvenient, timeconsuming, and costly, and does not provide a ‘real-world’ antennaimpedance estimation. Accordingly, there is a need to develop a methodto measure impedance seen by a transmitter in an antenna without havingto remove the transmitter from the network.

SUMMARY OF THE INVENTION

The present disclosure provides a method that can be used to measure theimpedance of an antenna while the transmitter is active.

In accordance with the present disclosure there is provided a method fordetermining impedance of an antenna coupled to an active radio frequencytransmitter. Voltage and current samples are received from a samplingprobe sampling a modulating broadcast signal being transmitted from thetransmitter to the antenna. Analog to digital conversion is performed ofthe voltage and current samples. Complex demodulation is performed ofeach of the voltage and current samples to baseband levels. Decimationof the voltage and current samples is performed to reduce the number ofsamples and an impedance estimate is calculated from the decimatedvoltage and current samples.

In accordance with the present disclosure there is also provided a radiofrequency transmitter comprising a modulation chain coupled to anantenna, the modulation chain modulating and amplifying a broadcastsignal for transmission through the antenna. A voltage probe and acurrent probe sample the modulating broadcast signal between themodulation chain and the antenna. One or more analog to digitalconverters convert voltage and current samples. A down conversion moduleperforms complex demodulation on the voltage and current samples andperforming decimation of the voltage and current samples to reduce thenumber of samples. A signal processing module calculates an impedanceestimate from the voltage and current samples.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present disclosure will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, as briefly described below.

FIG. 1 shows a block representation of a process for generating animpedance estimate in an active radio frequency transmitter.

FIG. 2 shows a representation of system for generating an impedanceestimate in an active radio frequency transmitter.

FIG. 3 shows a method of generating an impedance estimate in an activeradio frequency transmitter.

FIG. 4 shows a method of calculating an impedance estimate in an activeradio frequency transmitter using a fast Fourier transform.

FIG. 5 shows a method of calculating an impedance estimate in an activeradio frequency transmitter using time domain adaptive filtering.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

Embodiments are described below, by way of example only, with referenceto FIGS. 1-5.

The present disclosure is described with reference to digital radiobroadcasting; however, any type of wireless or radio frequencybroadcasting may utilize the system and method described herein.

The disclosed transmitter and method work with a modulating signal,rather than a fixed fundamental frequency or a training signal. Thedisclosure provides a device and method to generate an impedanceestimate using real audio being transmitted through the system as themeasurement is being made and still providing an accurate result. Sincevarious type of signal modulation and encoding schemes can be utilized,such as amplitude modulated (AM) signals, frequency modulated (FM)signals, National Radio System Committee NRSC-5A In-band/on-channelDigital Radio Broadcasting Standard compliant signals, Digital RadioMondiale (ETSI ES 201 980) compliant signals, or LORAN (Long Range Aidto Navigation) type system utilizing pulse signals, the frequencycontent available on any given update to the measurement is based onwhat content or audio is in the transmitted signal.

FIG. 1 shows a block representation of a process for generating animpedance estimate in an active transmitter of an antenna coupled to thetransmitter. The voltage 100 and current 101 samples are measured usingprobes such as a transformer-coupled current probe and a capacitiveattenuator voltage probe. The probes sample the modulating livebroadcast signal across the range of frequency content. The samplingrate does not need any particular relationship to the carrier frequencyof the modulated signal. Until the two voltage 100 and current 101samples are used to calculate the impedance, both the voltage 100 andcurrent 101 samples can be processed through independent parallel paths.The voltage 100 and current 101 are then sampled using analog-to-digitalconverters (ADC) 102 and 103 for each probe. Alternatively, the voltage100 and current 101 could be calculated from alternative samplescaptured using appropriate probes. For example, directional couplerscould be used to measure the forward and reflected voltage. In thiscase, voltage would be calculated by the sum of these samples and thecurrent determined by the difference in the samples. This calculationcan either be made on the analog samples, or at any point before thecomplex division 112.

In most cases, complex demodulation 104 and 105 is performed on eachsample in order to shift the samples 100 and 101 from radio frequencyback to baseband. This shift can be accomplished by multiplying acomplex phasor at the carrier frequency. Alternatively, if the carrierfrequency is low enough, the demodulation 104 and 105 can be bypassed.In this case, the resulting impedance measurement will be centeredaround baseband, rather than at the carrier frequency.

The signals are further processed by decimation at 106 and 107. Byperforming decimation at 106 and 107, the signals are passed through alow pass filter and then down-sampled to lower sampling rate. Bydecimating the signals, the computational requirements for thesubsequent execution is reduced, since both the number of samples andthe length of the discrete Fourier transforms (DFTs) to be implementedby the fast Fourier transforms are reduced. The amount of decimationrequired is dependent on the ADC bandwidth (number of bits) and samplingrate. A higher sampling rate will require more decimation of the samplesbefore the FFT is applied.

The impedance measurements are calculated by using blocks of samples.The block length determines the computational requirements and thefrequency of resolution that is obtained. For most antennas, theimpedance will typically not vary dramatically over the pass band, so ablock length that gives a reasonable number of sample points isappropriate. The sampling of these blocks will also affect the exacttime domain points used to determine the impedance. Representativesampling methods include an overlapping window approach, such as thatcommonly used for power spectral density estimations, and randomsampling. An overlapping window approach may be used in order to providea more accurate estimation of the relative frequency content of the twotime domain signals. Once the blocks of samples are obtained, it ispreferred that a window function 108 and 109 is applied to minimize theeffects of using finite length FFTs.

It is desirable to minimize the amount of spectral leakage to permitproper mapping of the impedance against frequency. This is aparticularly important parameter to control for AM (amplitudemodulation) broadcasting, since the large carrier would render anymeasurement made with a standard rectangular window essentially uselessat any frequency aside from the carrier. Appropriate window functions108 and 109 for this application include high-valued Kaiser windows,Blackmann-Harris window, Nuttall window, or similar low-resolution(high-dynamic-range) windows. However, based on the type of signal, anyone of a number of windows could be applied. For example, if the signalhas sufficient frequency content at all desired measurement frequencies,a window with less dynamic range may be used. Alternatively, thewindowing may be bypassed entirely if the signal permits a rectangularwindow to be used, although some degradation to the resulting impedancemeasurement may occur. Once the samples are processed through the abovewindow functions 108 and 109, they are passed through a discrete Fouriertransform (DFT) such as a finite Fourier transform (FFT) 110 and 111 toextract the phase and amplitude of each frequency component on both thevoltage and current samples for each block.

The fast Fourier transform (FFT) can be applied to extract the phase andamplitude of each frequency component; however, based on the type ofsignal, other methods can be employed to extract this information. Theresulting frequency domains for the voltage and the current can be usedto determine an approximate value for the impedance in which thetransmitter is feeding into. For example, the impedance at eachfrequency can be obtained by dividing the frequency domain value for thevoltage by the frequency domain value of the current (represented byblock 112). In other cases, such as when forward and reflected voltageis used to calculate the voltage and current, additional equations ordata manipulation may be required to obtain the impedance.

The accuracy of impedance measurement is highly dependent on the levelof frequency content present in the sampled block. Accordingly, theestimates must be averaged to obtain a reasonably accurate measurement.To achieve such an average, a first-order decaying exponential filterwith an adequate delay can be applied. Similar-type filters could beapplied to the estimates in order to achieve a reasonably accuratemeasurement.

Since the signal being used for the estimate is not a training signal orknown signal, there may be frequencies that have either intermittent orno signal content. If the impedance calculation is performed and thereis not enough signal content, then the estimate will be inaccurate atthose frequencies. In these cases, intelligent averaging andinterpolation 113 is applied to mitigate this effect. There are severaldifferent ways this can be accomplished. For example, if the signalintermittently has content at all frequencies, the impedance estimatecan be managed by making the weighting of the average proportional tothe signal power present in that block at each frequency. Using thismethod, the points with the most power at a particular frequency willhave the most influence on the impedance estimate at that frequency.However, situations will still arise where there is no power at all, atcertain frequencies. If there is no signal content at the frequency inquestion, and no reasonable estimate can be obtained for the impedance,a value can potentially be interpolated from other known points.

FIG. 2 shows a transmitter 200 providing impedance measurementcapability. A signal source 202 provides a broadcast or audio input tothe transmitter 200 and the modulation chain 201. A digital modulator204 modulates the incoming signal to the desired modulation. Themodulating signal is then up-converted by a synthesized frequency source206 to the desired carrier frequency. The resulting signal is thenfiltered by a band-pass filter 208, such as a dual band-pass filter, toremove unwanted frequency components. The signal is then amplified byone or more power amplifiers 210. A transmission line connects thetransmitter 200 to a tuning or load matching network 212 to provideimpedance matching with an antenna 214. The tuning network 212 istypically adjusted by using a test signal and lower power test equipmentto calculate an impedance estimate. In the present disclosure samples ofthe amplified signal prior in the transmission line to the tuningnetwork 212 are sampled using voltage and current probes 220 and 222.The probes may alternatively be directional couplers. The directionalcouplers measure the forward and reverse voltage waves in thetransmission line, whereas the voltage probe measures the voltage justat that point in the line, and the current probe measures the current atthat point. If the load is perfect (for example: resistive 50 ohm load)then the voltage and current measured at different lengths along thetransmission line would be the same. If not, a high and low points inboth voltage and current result, determined by the voltage standing waveratio. For most purposes, such as finding out power delivered to theload, or measuring the spectrum going to the antenna, the directionalsample is better, since all that is needed is the forward voltage tomake that measurement. In higher frequency (FM) systems directionalcouplers are more common than pure voltage or current samples, and aretypically coupled off using the geometry of the probe, rather than atransformer or capacitor. The relationship is that the forward voltageis the sum of the current and voltage samples at any point on thetransmission line, and the reverse voltage is the difference.

The voltage 220 and current 222 probe or coupler provide an analogoutput to analog to digital converters (ADCs) 224 and 226 respectively.The digital output is then demodulated using complex demodulation usingthe down-conversion module 228. Complex demodulation may be performedusing a processor such as a digital signal processor (DSP), fieldprogrammable gate array (FPGA) or application specific integratedcircuit (ASIC). The down-conversion module 228 can also providedecimation of the samples prior to passing through a low pass filter 230to remove the −2fc. The decimation process down-samples the demodulateddigital signals to a lower sampling rate. After filtering has beenperformed the remaining frequency data is processed by a signalprocessing module 232. Again, the signal processing module 232 may beimplemented by one or more processor, DSPs, FPGAs, or ASICs or acombination thereof. The signal processing module 232 can then implementprocessing of the samples such as by using a FFT to extract phase andamplitude of each frequency component for both the voltage and currentsamples for each block. Alternatively, the signal processing module 232may perform adaptive filtering using a finite impulse response (FIR),filter such as Least Mean Squares and zero padding the FIR result togenerate an impedance estimate. The resulting FIR structure produces afilter with the same amplitude and phase response as the antenna load.

The resulting impedance data can then be further processed by processor234, for either providing pre-distortion data to digital modulator 204,providing control of the tuning network or tuning values for use by thetuning network 212 to improve transmission efficiency and performance.Alternatively, the data may be provided to an external data processingsystem such as a computer 250 for further trending or storage on storagedevice 252. Again, this data may be utilized to control the loadmatching network 212. A computer or processor readable memory 236 may beused by the processors of down-conversion module 228, signal processingmodule 232 or processor module 234 to enable access to software code orinstructions, determine data value, configuration parameters oroperating characteristics. The memory 234 may be embodied in randomaccess memory, read only memory, non-volatile memory such as flashmemory.

FIG. 3 shows a method of generating an impedance estimate in an activetransmitter. The method commences with receiving voltage and currentsamples of a modulating signal 302 from a transmission line between atransmitter 200 and an antenna 214 using voltage and current probes 220and 222. The sampling is provided at a fixed sampling rate independentof the carrier frequency of the RF transmission. Analog to digital downconversion is then performed at 304 of voltage and current samples byADCs 224 and 226. The samples may be converted in parallel orsequentially depending on the implementation of the ADC converters.Complex demodulation is then performed on the digital samples at 306 bydown-conversion module 228. The demodulated samples are decimated at 308to reduce the number of samples to be processed. The estimate may alsobe filtered using a low pass filter 230. The impedance estimate can bygenerated by performing impedance calculations using the voltage andfrequency samples by signal processing unit 232. The impedance estimatemay be determined using frequency domains for the voltage and currentsamples as described in FIG. 4 or by determining a frequency responsebetween current and voltage samples using a time domain adaptivefiltering method as described in FIG. 5. The impedance estimate can thenbe provided to internal processor 234 or external computing device 250at 312 for further processing or storage. The resulting estimate maythen be provided to load matching network or digital modulator 204 toimprove performance of the transmitter 200.

FIG. 4 shows a method of calculating an impedance estimate in an activetransmitter using a fast Fourier transform (FFT) as implemented at 310of FIG. 3. The signal processing unit 232 at 410 can, after decimationand filtering is performed at 308, apply a windowing function foraveraging across the samples based upon the type of transmitted signal.Windowing is applied to shape the time portion of sample data, tominimize edge effects that result in spectral leakage in the FFTspectrum. By using window functions correctly, the spectral resolutionof the frequency-domain result will increase. Windowing is required whenusing a broadcast signal due to the large carrier frequency component.The modulating signal content is typically anywhere from 15 to 50 dBlower, so getting an accurate measurement without it would beunfeasible.

Appropriate windowing functions for this application include high-valuedKaiser windows, Blackmann-Harris window, Nuttall window, or similarlow-resolution (high-dynamic-range) windowing techniques. However, basedon the type of signal, any one of a number of windowing techniques couldbe applied. Similarly windowing may be bypassed if it is not requiredfor the signal being analyzed. A FFT calculation is then performed onthe data to extract the phase and amplitude of each frequency componenton both the voltage and current samples at 412. Complex division of V(f)and V) can then be performed to generate the impedance estimates for themodulating signal at 414. With certain types of broadcast programs,especially voice content, this can vary by large amounts (40 to 50 dB)from one batch of samples to the next, this is where the “intelligentaveraging” can be applied at 416. By weighting the average according tothe amount of energy in each bin, a much more accurate result isobtained. Interpolation can then be applied for missing frequencies at418 if no data is available and to mitigate effects of using a livesignal for estimation.

FIG. 5 shows a method of calculating an impedance estimate in an activetransmitter using time domain adaptive filtering as implemented at 310of FIG. 3. As an alternative to the approach using FFTs of the currentand voltage signals, the frequency response between current and voltagecan be determined using a time domain adaptive filtering method. Afterdecimation at 308, the current samples and a delayed version of thevoltage samples are passed into an adaptive finite impulse response(FIR) filtering process, such as Least Mean Squares (LMS) at 510. Otheradaptive FIR algorithms could also be used. The current samples aretaken to be the input to this process, with the delayed voltage samplesrepresenting the desired signal. As the filtering process adapts theresulting FIR structure, it will produce a filter with the sameamplitude and phase response as the load, with some additional delay.The adaptive filtering process is inherently averaging, so the impedancemeasured by this method would not require any further averaging. Oncethe filter has been determined, the impedance estimate of the load canbe determined by zero-padding the filter and taking a DFT (or FFT) at512. This provides a complex response of the load versus frequency. Theadditional delay added to the voltage vector in the adaptive filteringprocess must be removed at 514 to get an accurate phase measurement.This can be accomplished by converting the delay to the appropriatephase at each frequency and canceling it. The resulting estimate canthan be provided at 312.

It will be understood that a combination of hardware and softwarecomponents, with some components being implemented by a given functionor operation of a hardware or software system, and many of the datapaths illustrated being implemented by data communication within acomputer application or operating system. In addition, numerousmodifications thereto will appear to those skilled in the art.Accordingly, the above description and accompanying drawings should betaken as illustrative and not in a limiting sense. It will further beunderstood that it is intended to cover any variations, uses, oradaptations following, in general, the principles of the disclosure andincluding such departures from the present disclosure as come withinknown or customary practice within the art to which the disclosurepertains and as may be applied to the essential features herein beforeset forth, and as follows in the scope of the appended claims. Acomputer readable memory may be utilized to store instructions of thedescribed method for execution by a processor, DSP or ASIC.

What is claimed is:
 1. A method for determining impedance of an antennacoupled to an active radio frequency transmitter, and involving use ofthe transmitter for such impedance determination, the method comprising:receiving voltage and current samples across a range of frequencycontent from a sampling probe sampling a modulating broadcast signalbeing transmitted from the transmitter to the antenna; performing analogto digital conversion of the voltage and current samples; performingcomplex demodulation of each of the voltage and current samples tobaseband levels; performing decimation of the voltage and currentsamples to reduce the number of samples; calculating an impedanceestimate from the decimated voltage and current samples; and removingdelay by converting the delay to the appropriate phase at each frequencyand cancelling same.
 2. The method of claim 1, wherein calculating theimpedance estimate further comprises: performing windowing and averagingacross a plurality of voltage and current samples; performing a fastFourier transform of the plurality of voltage and current samples; andperforming complex divisions of V(f) and I(f) to calculate the impedanceestimates.
 3. The method of claim 2, wherein said windowing is selectedfrom the group consisting of: high-valued Kaiser window,Blackmann-Harris window, Nuttall window, and high-dynamic-rangewindowing functions.
 4. The method of claim 1, further comprising usingthe calculated impedance estimate to adjust a matching network coupledto the antenna.
 5. The method of claim 1, wherein the broadcast signalcomprises at least one of an amplitude modulated (AM) signal, afrequency modulated (FM) signal, a National Radio System CommitteeNRSC-5A In-band/on-channel Digital Radio Broadcasting Standard compliantsignal, a Digital Radio Mondiale (ETSI ES 201 980) compliant signal, anda LORAN (Long Range Aid to Navigation) compliant signal.
 6. A radiofrequency transmitter comprising: a modulation chain coupled to anantenna, the modulation chain modulating and amplifying a broadcastsignal for transmission through the antenna; a voltage probe and acurrent probe for sampling the modulating broadcast signal across arange of frequency content between the modulation chain and the antenna;one or more analog to digital converters for converting voltage andcurrent samples; a down conversion module adapted to (i) perform complexdemodulation on the voltage and current samples, and (ii) performdecimation of the voltage and current samples to reduce the number ofsamples; and a signal processing module for calculating an impedanceestimate from the voltage and current samples; wherein the radiofrequency transmitter is adapted to remove delay by converting the delayto the appropriate phase at each frequency and cancelling same.
 7. Theradio frequency transmitter of claim 6, wherein the signal processingmodule is adapted to: (a) perform windowing and averaging across aplurality of voltage and current samples; (b) perform a fast Fouriertransform of the plurality of voltage and current samples; and (c)perform complex divisions of V(f) and I(f).
 8. The radio frequencytransmitter of claim 7, wherein said windowing is selected from thegroup consisting of: high-valued Kaiser window, Blackmann-Harris window,Nuttall window, and high-dynamic-range windowing functions.
 9. The radiofrequency transmitter of claim 6, wherein the modulation chain furthercomprises: a digital modulator to modulate a received broadcast signal;a synthesized frequency source adapted to up-convert the signalmodulated by the digital modulator; a band-pass filter adapted to filterthe up-converted broadcast signal; and an amplifier coupled to theantenna and adapted to transmit the broadcast signal.
 10. The radiofrequency transmitter of claim 9, wherein a tuning network is coupledbetween the transmitter and the antenna, and wherein the impedanceestimate is used for adjusting the tuning network.
 11. The radiofrequency transmitter of claim 6, wherein the voltage and current probescomprise a transformer-coupled current probe and a capacitive attenuatorvoltage probe.
 12. The radio frequency transmitter of claim 6, whereinthe broadcast signal comprises at least one of an amplitude modulated(AM) signal, a frequency modulated (FM) signal, a National Radio SystemCommittee NRSC-5A In-band/on-channel Digital Radio Broadcasting Standardcompliant signal, a Digital Radio Mondiale (ETSI ES 201 980) compliantsignal, and a LORAN (Long Range Aid to Navigation) compliant signal.